Preliminary treatment of sewage is the first step in the sewage treatment process. It involves the removal of large solids, oils, greases, and other debris that can cause damage to equipment or hinder the treatment process. The correct answer to the given question is option 'D', which states that all of the above statements are correct. Let's discuss each statement in detail:

1. Oils and greases are removed by skimming tanks:
- Skimming tanks, also known as oil and grease traps or interceptors, are designed to remove oils, greases, and other floating substances from the sewage.
- These substances are lighter than water and tend to float on the surface. Skimming tanks are designed to allow the floating oils and greases to rise to the top, where they can be skimmed off and removed from the sewage.

2. Floating materials are removed by screening:
- Screening is done to remove large floating materials such as sticks, rags, plastics, and other debris that may be present in the sewage.
- Screens with small openings are used to trap and remove these materials. The trapped materials are then collected and disposed of properly.

3. Grit and sand are removed by grit chambers:
- Grit chambers are designed to remove heavy materials such as sand, gravel, and small stones from the sewage.
- These materials settle to the bottom of the grit chamber due to their weight, while the lighter organic matter remains suspended in the sewage.
- Grit chambers are typically equipped with mechanisms like grit pumps or grit classifiers to remove the settled grit for proper disposal.

By implementing these three processes during preliminary treatment, the sewage is effectively prepared for further treatment in the secondary and tertiary stages of the sewage treatment process. Removing oils, greases, floating materials, and grit ensures that the downstream treatment processes are not hindered and the overall efficiency of the sewage treatment plant is maximized.

In conclusion, the correct answer is option 'D' - all of the above statements are correct. Oils and greases are removed by skimming tanks, floating materials are removed by screening, and grit and sand are removed by grit chambers during the preliminary treatment of sewage.